Topic - HowTo "mix" two materials using an image map

what I want to achieve is to mix two materials together using an image map. Let's say a painted surface with some corrosion spread over it. The "mixer" is a grayscale image which would blend material 1 and material 2 based on the grayscale values. white takes all of material 1, black takes all of material 2 and the grayscales between interpolate to a value between.

Of course I could produce a final image with PS as I did it here, but it wonder if this could be done directly in modo.